Rectus femoris: the quadriceps muscle that crosses the hip and the knee

What it is

The only part of quadriceps femoris that crosses both the hip and knee.

Location

Originates from the anterior inferior iliac spine (straight head) and the ilium above the acetabulum (reflected head), and inserts through the quadriceps tendon onto the patella and via the patellar ligament onto the tibial tuberosity.

Function

Extends the leg at the knee and flexes the thigh at the hip.

Body system

Muscular · paired (left and right)

FMA ID

FMA38928 (Foundational Model of Anatomy)

Key facts

• Innervation: femoral nerve (L2–L4)
• Blood supply: lateral circumflex femoral artery

Clinical relevance

Avulsion of its origin from the anterior inferior iliac spine is a recognized sports injury, especially in adolescents.

Rectus femoris has two heads of origin on the ilium. The direct (straight) head arises from the anterior inferior iliac spine (AIIS). The indirect (reflected) head arises from the superior acetabular ridge, the upper rim of the hip socket. The two merge about 1 cm below their origins into a conjoined tendon. (Radiopaedia: Rectus femoris muscle.)

Radiopaedia notes that about 15% of the muscle fibres intermingle between the two origins, so the muscle can be thought of as two muscles packaged together. (Radiopaedia: Rectus femoris muscle.)

Some anatomists describe a third, capsular head. It arises from the front of the hip joint capsule (the iliofemoral ligament). In cadaver studies summarised by Armstrong and colleagues it was present on both sides in about 83% of specimens and measured roughly 2 cm long and 4 cm wide. It is not part of standard textbook descriptions. (Armstrong T, Pass B, O'Connor P. The capsular head of the proximal rectus femoris muscle. Br J Radiol. 2022.)

Rectus femoris is a biarticular (two-joint), fusiform (spindle-shaped) muscle and the most superficial part of quadriceps. Below the conjoined tendon, the indirect head continues deep into the muscle belly as an internal tendon called the central septum, which forms the deep part of the upper musculotendinous junction. (Armstrong T, Pass B, O'Connor P. The capsular head of the proximal rectus femoris muscle. Br J Radiol. 2022.)

At its lower end, the muscle becomes a short free tendon that joins the tendons of the three vasti to form the quadriceps tendon. (Radiopaedia: Rectus femoris muscle.)

The University of Washington atlas places its insertion on the base of the patella, where it forms the central part of the quadriceps tendon. (University of Washington Musculoskeletal Atlas: Rectus femoris.)

The quadriceps tendon inserts into the patella and continues below it as the patellar ligament, which attaches to the tibial tuberosity. The force of rectus femoris therefore reaches the tibia through the patella. (OpenStax Anatomy and Physiology 2e, 11.6: Appendicular Muscles of the Pelvic Girdle and Lower Limbs.)

Because it crosses the front of both joints, rectus femoris flexes the thigh at the hip and extends the leg at the knee. The three vasti cross only the knee, so they extend the knee but do not move the hip. (Radiopaedia: Rectus femoris muscle.)

Crossing two joints has a cost. Armstrong and colleagues explain that crossing the hip and knee and having a high proportion of fast-twitch (type II) fibres makes rectus femoris prone to stretch injury. (Armstrong T, Pass B, O'Connor P. The capsular head of the proximal rectus femoris muscle. Br J Radiol. 2022.)

Hip flexor length, of which rectus femoris and iliopsoas are the main targets, is often checked with the modified Thomas test. The person lies back on the edge of a table holding one knee to the chest while the other leg hangs down, and the examiner checks whether the hanging thigh can reach the horizontal (0 degrees of hip extension) with the pelvis in a neutral position. (Eimiller K et al. Reliability of Goniometric Techniques for Measuring Hip Flexor Length Using the Modified Thomas Test. Int J Sports Phys Ther. 2024.)

Rectus femoris is supplied by muscular branches of the femoral nerve, like the rest of the anterior compartment of the thigh. (University of Washington Musculoskeletal Atlas: Rectus femoris.)

Its arterial supply is the descending branch of the lateral circumflex femoral artery. (Radiopaedia: Rectus femoris muscle.)

TeachMeAnatomy describes a simple bedside test of the femoral nerve that uses the quadriceps: with the knee slightly bent, the patient straightens the leg against resistance. No contraction suggests femoral nerve damage. (TeachMeAnatomy: Muscles of the Anterior Compartment of the Thigh.)

The four quadriceps muscles sit around the femur. Rectus femoris lies on the front of the thigh, vastus lateralis on the outer side, vastus medialis on the inner side, and vastus intermedius between them, directly deep to rectus femoris. (OpenStax Anatomy and Physiology 2e, 11.6: Appendicular Muscles of the Pelvic Girdle and Lower Limbs.)

The key difference is origin. The vasti all arise from the femur (greater trochanter, intertrochanteric line, linea aspera and shaft). Rectus femoris is the only quadriceps muscle that arises from the pelvis (the anterior inferior iliac spine and the area above the acetabulum), which is why it is the only one that crosses two joints. (TeachMeAnatomy: Muscles of the Anterior Compartment of the Thigh.)

All quadriceps, adductor and hamstring muscles with a comparison table. Thigh muscles.

The upper (proximal) rectus femoris is among the most frequently injured muscles in athletes, especially in kicking sports such as football (soccer). Injuries can affect either tendon of origin, the conjoined tendon, the central septum or the junction between muscle and tendon. Tears of the tendons themselves, or of bone, are rare in adults. (Armstrong T, Pass B, O'Connor P. The capsular head of the proximal rectus femoris muscle. Br J Radiol. 2022.)

Radiopaedia lists the range of athletic injuries by site: AIIS avulsion, injury to the direct or indirect head or the free tendon, proximal myotendinous junction injury, intramuscular degloving injury, and distal myotendinous junction injury. (Radiopaedia: Rectus femoris muscle.)

In adolescents, the straight head can instead pull off a fragment of bone from the anterior inferior iliac spine (an AIIS avulsion fracture). These injuries occur mostly between 14 and 17 years of age, slightly more in males, and most often in kicking sports. The mechanism is forced hip extension with the knee flexed. Almost all cases are managed without surgery. (Radiopaedia: Anterior inferior iliac spine avulsion injury.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Descending branch of lateral circumflex femoral artery · cardiovascular
• Sartorius · muscular
• Vastus intermedius · muscular
• Femur · skeletal
• Vastus lateralis · muscular
• Adductor magnus · muscular
• Vastus medialis · muscular
• Femoral vein · cardiovascular
• Femoral artery · cardiovascular
• Adductor longus · muscular